1. Field of the Invention
This invention relates to power amplifiers, and more specifically to transistorized power amplifiers provided with a compression effect similar to that found in vacuum tube amplifiers.
2. Background Art
Tube type power amplifiers using grounded cathode push-pull designs are well known in the art. In such amplifiers, operated in class AB or class B, for example, where coupling capacitors are used to provide an input signal to the two grids, a phenomenon termed `tube-compression` is known and appreciated by musicians and other users.
The compression effect results from the overload clipping characteristic of the tube amplifier, in which excessive input drive signals cause the grids of the tubes to be positively biased. For such inputs, in which the peak signal is higher than the negative bias voltage applied to the tubes, grid current may be drawn, raising the biasing voltage stored on the coupling capacitors. The increased capacitor voltage leads to cross-over distortion in the amplifier output, due to cutoff of tube operation because of increased negative bias by the capacitors.
The relatively soft cut-off characteristic of a vacuum tube, however, provides a tolerable degree of "soft" cross-over distortion in which the cross-over region of a sinusoidal signal, for example, exhibits a reduced slope, but not a zero slope which would result in harsh, intolerable cross-over distortion. The charging of the coupling capacitors, however, also reduces the overall signal level, thus reducing the clipping distortion of the amplifier plate circuit.
The tube type amplifier thus provides a reduced plate clipping characteristic, coupled with soft cross-over distortion due to high level input signals. The combined effects, due to the use of coupling capacitors in the push-pull configuration and the tube characteristics, provide the output compression characteristic previously mentioned.
An attempt to achieve similar reduced clipping and compression characteristics in solid state, transistorized amplifiers by the use of coupling capacitors at the inputs to a pair of common emitter transistors in push-pull configuration is typically unsuccessful, however. Inasmuch as transistors are inherently current amplifying devices, even a slight increase in negative input bias voltage manifested on the coupling capacitors leads to harsh cross-over distortion. Thus, as the input level increases until the occurrence of transistor saturation, the coupling capacitor becomes charged by the increased base current drawn during saturation. When the input level drops, or becomes negative, the transistor, no longer in saturation, cannot provide the necessary levels of base current to discharge the capacitor. The voltage applied to the transistor base cannot keep the transistor in its linear, active region, and the transistor cuts off for such low levels or negative levels of input. The cutoff is harsh, in which a dead-zone, having a zero slope portion, is provided between halves of a sinusoidal signal, for example. See E. A. Angelo, Jr., Electronic Circuits, 2d Ed., McGraw-Hill, New York, 1964 at section 11-3, for example.
Prior art devices for reducing cross-over distortion are disclosed in U.S. Pat. Nos. 4,220,930; 4,160,216; 4,237,425 and 3,564,445. A direct coupled push-pull amplifier with a biasing arrangement for linearizing the cut-off characteristics of the output is disclosed in U.S. Pat. No. 3,371,286.
None of the prior art, however, discloses structure for simulating the compression characteristics of a vacuum tube power amplifier in a transistor amplifier.